First of all, thank you for all your replies. Considering that both Toady and Footkerchief replied to this thread, I guess that the first answers are somehow validated :-)
When I mentioned open source, I was thinking in options to see some help to Toady, considering the complexity of DF; the
huge additions that are planned for future releases; that each time someone add a feature to a program at least one hundred bugs appear, and that this game is developed by one single person (my compliments for Toady for the courage in coding it).
My interest behind Toady's intentions is because I'm a lawyer and lately I've been thinking about writing a paper about the Economical Viability of Community developed software focusing on copyrights, and he probably may have a strong opinion on it.
I don't find anything wrong with his decision, but I find it hard to maintain the creativity behind this project without having to waste hours and hours finding bugs of recent implemented features. In this scenario, it seemed logical to me i) to open part or the whole source with a restrictive license (as explained by Urist McDepravity), ii) allowing a strict group of old community members to peek at the code to help hunt for bugs or iii) simply hire someone to help.
Even though I know that once the source is opened, nothing stops someone else to do whatever he wants with it, it seems to me that DF has already a strong community behind its back and I find it difficult to imagine people switching to a fork, mainly because the creative force behind the DF development would still be Toady (and that's something infungible).
In this scenario, it would be morally and legally plausible to still ask for donations, even though there are more people helping coding it - it kinda makes me remember that Canonical still earn some bucks out of Ubuntu, even though it is open source. And I imagine that faster development and bug fixing would be positive enough to attract more donors.
On the long term, it is always possible for someone else to start a similar project helped by a community - it may not have worked well recently, but if you see the history of open source software, most projects started just like that.
Well, these are only ramblings out of my desire to see this project to its fullest. I totally support whatever decision the developers maintain and don't want to cause a flame war (if so, please, just delete this post). As I said, I was just curious about the motivations from the developers. Until them, the most I can do is to find a way to donate via paypal.